Modular building assembly systems have long been available to for the construction and erection of various structures such as office cubicles, industrial work stations, and scaffolding. Such modular building assembly systems usually have some type of standard beam that can be joined to other beams and to which various accessories can be attached. Solid bars, of circular or regular polygonal shape (such as square or hexagonal) may be used, but are inferior to tubes of the same shape because tubes have a better resistance to torsion for the same mass of material than do solid bars. Circular or regular polygons lack an easy point of attachment for accessories and other beams, so more complex shapes are preferred. One such complex shaped beam is a cruciform beam (see U.S. Pat. No. 5,481,842 to Gautreau, FIG. 1). The cruciform beam comprises a center tube surrounded by four angle bars arranged in a square pattern in cross-section and each joined to the center tube with a web or fin, the fins forming a cross when the beam is viewed in cross-section. Accessories can be attached along the cruciform beam by clamping the accessory to one of the angle bars or in a longitudinal groove defined by the spaces between the fins and angle bars. The cruciform beam is relatively strong in resisting buckling when torsion is applied to the beam around an axis orthogonal to the long axis of the cruciform beam because in cross-section, a substantial amount of the beam material is distant from the center longitudinal axis. Such torsion occurs when the cruciform beam spans a space and a load is attached to the beam somewhere in the middle. However, the cruciform beam is not relatively strong when torsion is applied around the long axis of the cruciform beam. Such torsion occurs when a load is cantilevered from the side of the cruciform beam. Since a cruciform beam for a given size and weight does not have good resistance to torsion around its long axis, accessories are usually not cantilevered from the side of the cruciform beam.
What is needed is a modular building system with a beam that has strong resistance to torsion around its long axis.